1. Field of the Invention
The present invention relates to free-space-optical (FSO) communications. In particular, the present invention relates to an optical head for an airborne FSO communications system. And in more particularity, the present invention relates to a three-tier steering design for an optical head of an airborne FSO communications system.
2. Background of the Invention
As data-throughput requirements increase, the ability to transmit vast amounts of data requires a flexible and secure communications network with very high bandwidth capability. The Department of Defense (DOD) has established several programs to develop such technology. For example, the TeraHertz Operational Reachback (THOR) program has been established to find and demonstrate new ways to handle the DOD's increasing data needs in mobile environments. The objective of the THOR program is to provide secure, assured, high-data-rate, and end-to-end communications to airborne, terrestrial, and surface war fighters.
Free Space Optical (FSO) communications is an emerging technology which will play a major role in accomplishing several aspects of the DOD's objectives. In conventional applications, FSO communications refers to the transmission of modulated infrared (IR) laser beams through the atmosphere to obtain optical communications. Like fiber-optic communications, FSO uses lasers to transmit data, but instead of enclosing the data stream in a glass fiber, it is transmitted through the air or space. To accomplish the transmission, FSO transmits laser beams from one optical head to another optical head which receives the laser beams on highly sensitive photon-detector receivers. To date, FSO communications systems have been statically based-systems mounted between buildings which are subject to minimal movement and vibration.
A more complex and challenging environment is presented, however, when FSO technology is utilized in mobile airborne applications. Airborne FSO communications requires low-jitter pointing, acquisition, and tracking (PAT) solutions. In this environment, the amplitude and frequency of the jitter is platform dependent and dependent on the placement of the aperture with respect to the leading edge of transition to turbulence. For example, concepts of operations (CONOPS) relative to the THOR program require the pointing jitter of a beam of about 100 μrad divergence to be less than about 10 μrad of jitter, which is a generally accepted requirement for pointer-tracker (P/T) subsystems in FSO communications systems. That is to say, it is a requirement for pointer-tracker subsystems in FSO communications systems to point within 1/10 of the beam divergence. For example, if the beam divergence during tracking is 100 μrad, to satisfy the link budget, then the system must point to within 10 μrad (i.e., achieve jitter of less than 10 μrad). However, this jitter specification is beyond the capability of most available airborne pointer-trackers.
One way of dealing with airborne-platform vibration is to inertially stabilize the pointer-tracker (P/T), by developing larger and heavier systems, or by applying active vibration isolation, or both. The only known pointer-tracker systems that can achieve the aforementioned jitter requirement are expensive and heavy inertially-stabilized systems utilized on larger platforms. This solution, which utilizes coarse and fine steering elements in conjunction with sophisticated inertial stabilization, is too large to install on an unmanned aerial vehicles (UAV's), for example, and it generally does not have the agility needed to quickly (i.e., within approximately 100–200 msec) acquire new link partners in FSO communications systems. The active-isolation designs may be another viable solution for some platforms, but they have difficulty matching the higher frequencies of acoustic vibration on other platforms. And neither the heavy design nor active-isolation design approaches can mitigate aero-optic jitter induced by the boundary layer about the airborne platform.
The aforementioned aero-optic problem relates to the phase error induced by the compressible shear layer close to the airborne platform. That is, while Cn2 values may be quite small for platforms at 30,000 ft, indicating that open-atmosphere disturbances are of little consequence, research indicates that for platforms having a Mach number of about 0.8 or greater, a compressible shear layer will be convecting and evolving past the aperture of the platform that will produce beam jitters on the order of 100 μrad, and significant Strehl-ratio degradation, depending on beam diameter. It is the approximate 100 μrad of jitter, at approximately 1 kHz, along with the known vibration characteristics of several existing platforms (such as UAV's and larger, high-flying platforms), that combine to challenge the 10 μrad jitter requirement of the airborne FSO-communications mission.
A factor that facilitates the mitigation of the jitter problem is the “cooperative” beacon in the form of the link partner. Since the system can rely on this strong positioning signal, the control loop, if fast enough, could continually update steering to account for jitter. That is, full jitter correction would negate the need for inertial stabilization, either through increased mass or through active isolation. Developing a fast enough system is a challenge, however, because fast-steering mirror based designs (FSM's) are not available to provide +/−100 μrad steering at 2–5K kHz, while still steering over several degrees at +/−at 500 Hz. FSM's are utilized in a broad range of line-of-sight stabilization, laser communication, jitter motion compensation, military-targeting systems, optical instrumentation and high-energy laser-pointing applications. The temporal frequencies of the jitter in the compressible-flow regime could be several kHZ, which is beyond the close-loop response of conventional, fast-steering-mirror based (FSM) designs. For example, there is no known FSM available that can provide the conventional steering of approximately +/−3° with a command response of approximately 500 Hz, while still keeping up with jitter (optical tilt) corrections of +/−100 μrad at a 2–5 kHz response.
While other missions, commercial (i.e., airborne imaging of sporting events) and military (i.e., IRCM, targeting), rely today on two-tier pointer-tracker systems, with coarse steering and fine steering anchored to a fine track sensor (FTS), the airborne FSO communications mission requires higher pointing resolution, and further reduction of jitter, while resident in an especially unfriendly environment. So, while the aforementioned prior art comprises of commercially available pointer-tracker systems using coarse and fine steering, these systems in general will not adequately perform in the airborne FSO communications mission.
It would be advantageous and desirable to provide an airborne free space optical (FSO) communications system which overcomes the aforementioned disadvantages of the present airborne FSO communications systems. In particular, it would be beneficial to provide an airborne FSO communications system having low-jitter pointing, acquisition, and tracking (PAT) characteristics. Moreover, it would be advantageous to satisfy the 10 μrad jitter requirement of the airborne FSO based communications mission. For example, it would be advantageous to provide a system with a pointing jitter being less than about 10 μrad for a beam having approximately 100 μrad divergence. Furthermore, it would be desirable to provide conventional steering of ˜+/−3° with a command response of ˜500 Hz, while still keeping up with jitter (optical tilt) corrections of +/−100 μrad at a 2–5 kHz response. Moreover, it would be desirable to provide a lightweight and less expensive optical head for an airborne FSO communications system which mitigates vibration-induced and aero-optic induced jitter, and as a result, is capable providing a highly refined pointing resolution to the pointer-tracker (P/T) system.